I can't see what your drain plug looks like, but my general advice would be to make the minor investment in a new drain plug when you go with the "sealer" route - and topsider in the future.
There's lots of things you can try. Loctite is pretty good for a solution that you can back out of later. The Permatex product that aTOMic mentioned sounds good. Teflon tape might work too - all good suggestions.
If you want a seal that will never leak, but would be pretty difficult to back out of - use JB Weld epoxy.
The problem a lot of people get into with these drain plugs is rounding off the fairly soft head, rather than stripping the threads. So, I'd have to recommend to everyone - use a 6-pt socket, and follow the torque recommendation carefully.